1. Field of the Invention
The present invention relates to an automatic focus adjusting mechanism for focus adjustment, and to an optical image acquisition apparatus with the automatic focus adjusting mechanism, which is used to acquire an optical image of a pattern formed on a test sample such as a mask.
2. Description of the Related Art
In optical apparatuses, such as lithography apparatuses for transferring a circuit pattern to a semiconductor substrate, and defect testing apparatuses for testing a semiconductor integrated circuit pattern, it is necessary to acquire an optical image with high resolution equal to or less than the circuit pattern size. To acquire an optical image with such high resolution, an automatic focus adjusting mechanism is indispensable, which can keep constant the distance between a test sample, such as a semiconductor substrate, and an object lens, and can focus on the test sample. There is a demand for enhancing the accuracy of the automatic focus adjusting mechanism.
There exist various types of automatic focus adjusting mechanisms, and JP-A No. 6-102011 (KOKAI), for example, discloses an automatic focus adjusting mechanism of an optical lever scheme. In this scheme, a light beam is obliquely applied to the surface of a test sample, and the light reflected therefrom is detected by a light-receiving sensor. Focus adjustment is performed utilizing the fact that the output of the light-receiving sensor varies depending upon the surface position of the test sample along the optical axis. Namely, the detection output contains information indicating the surface position of the test sample, and hence a focus error signal corresponding to the surface position is extracted from the detection output. The focus error signal is fed back to a control circuit, and the control circuit controls the position of a table, on which the test sample is placed, to an image pickup position (a so-called focal position) with high accuracy. As a result, the test sample has its surface position adjusted so that an image pickup optical system as a main optical system can be focused on the test sample with high accuracy.
However, this scheme requires a light source for generating a focusing laser beam, in addition to a light source for picking up pattern images, which inevitably makes the optical system complex. Further, in accordance with increases in the degree of complexity of the optical system, the cost of the system will inevitably be increased. This also sets restraints on designing the optical system. If a light source of a wavelength longer than that of the pattern pickup light source is used to suppress the cost, the difference in wavelength may lead to a focus adjustment error.
JP-A No. 5-297262 (KOKAI) discloses an automatic focus adjusting mechanism of a differential pinhole scheme, and JP-A Nos. 2007-306013 and 2007-225311 (KOKAI) disclose automatic focus adjusting mechanisms of a differential slit scheme. In these schemes, a light beam is emitted to the surface of a test sample, and the light beam reflected therefrom is split into two segment light beams by a beam splitter. The amount of each segment light beam is detected, and the focus is adjusted based on the detected light amounts.
Specifically, in the focus adjusting method disclosed in JP-A No. 2007-225311, a thin rectangular slit image is projected on the surface of the test sample placed on a mount table, and the light beam reflected therefrom is collected by an object lens and an image reconstructing optical system. The reflected light beam is split into first and second segment light beams by a beam splitter. Across the optical path (first optical path) corresponding to the first segment light beam, a focus adjusting aperture and a light receiving unit are provided closer to the test sample than the image forming plane on which a slit image is formed by an image forming lens. Further, across the optical path (second optical path) corresponding to the second segment light beam, a focus adjusting aperture and a light receiving unit are provided remoter from the test sample than the image forming plane. Each focus adjusting aperture is formed in a thin rectangle of an appropriate width. When a focus error has occurred, the slit image on the light receiving unit provided across one of the first and second optical paths is widened, while the slit image on the light receiving unit provided across the other optical path is narrowed. As a result, one of the signals output from the light receiving units provided across both optical paths has a higher level than the other signal. By adjusting the position of the mount table to make equal the amounts of light detected by the light receiving units for focal adjustment, the object lens can be focused on the test sample.
The focus adjusting method disclosed in JP-A No. 2007-225311 does not require a light source that differs from the light source used for picking up an image of the test sample. Further, in the above-mentioned schemes, if no patterns are formed on a mask, or if a relatively rough pattern is formed on the mask, the focus adjusting apertures are set across the first and second optical paths to include therein the major part of the widthways portion of a light amount distribution corresponding to the slit image. As a result, a focus error can be detected with high accuracy.
However, in this method, it is possible that the slit image projected on the test sample (mask surface) will be subjected to diffraction effect due to a mask pattern formed on the test sample, whereby the light amount distribution on the focus adjusting aperture plane will be scattered to make it difficult to detect the focus. This problem will be conspicuous if a very fine pattern is formed on the test sample, since the very fine pattern causes serious diffraction effect.
As described above, when a fine pattern is formed on a test sample, the light beam reflected therefrom will be diffracted. The diffraction will cause the light amount distribution of a slit image on the focus adjusting aperture plane to vary and become wider. In particular, in an automatic focus adjusting mechanism using a focus adjusting aperture stop with a rectangular aperture, the light amount detected by a focal adjustment light receiving unit is extremely reduced. Further, if the light amount distribution at the edge of the aperture formed in the focus adjusting aperture stop is reduced, correct focus error detection cannot be realized. These problems will be caused by diffraction effect of a pattern formed on the surface of a test sample. Since test samples have different patterns, changes in light amount due to diffraction cannot be estimated beforehand. In view of the above-mentioned problems, there is a demand for development of an automatic focus adjusting mechanism capable of reliably detecting a focus error regardless of the pattern formed on the surface of a test sample, and development of an optical image acquisition apparatus equipped with the automatic focus adjusting mechanism.